Tunable Electron Donating and Accepting Properties Achieved by

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Tunable Electron Donating and Accepting Properties Achieved by Modulating the Steric Hindrance of Side Chains in A-D-A Small-molecule Photovoltaic Materials Delong Liu, Liyan Yang, Yang Wu, Xiaohui Wang, Yan Zeng, Guangchao Han, Huifeng Yao, Sunsun Li, Shaoqing Zhang, Yun Zhang, Yuanping Yi, Chang He, Wei Ma, and Jianhui Hou Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b03142 • Publication Date (Web): 16 Dec 2017 Downloaded from http://pubs.acs.org on December 17, 2017

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Chemistry of Materials

Tunable Electron Donating and Accepting Properties Achieved by Modulating the Steric Hindrance of Side Chains in A-D-A Smallmolecule Photovoltaic Materials Delong Liu, , Liyan Yang, , Yang Wu, Xiaohui Wang, Yan Zeng, , Guangchao Han, , Huifeng Yao,†,‡ Sunsun Li,†,‡ Shaoqing Zhang,†,‡ Yun Zhang,†,‡ Yuanping Yi,‖,‡ Chang He,*,†,‡ Wei Ma,*,§ and Jianhui Hou*,†,‡ †‡

†‡

§

§

‖‡

‖‡

†

Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China §

State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049, P. R. China

‖

Beijing National Laboratory for Molecular Science, Key Laboratory of Organic Solids Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China ‡

University of Chinese Academy of Sciences, Beijing 100049, P. R. China

ABSTRACT: Modulation of the electron donating and accepting properties of organic semiconductors is an important topic in the field of organic photovoltaics. Although the small-molecule semiconductors (SMs) with an A-D-A structure can act as either electron donor or acceptor in organic photovoltaic (OPV) devices, the reason why molecules with similar conjugated structures play different roles remains unclear. In this work, we designed and synthesized two A-D-A SMs named BTCN-O and BTCN-M, which have an identical backbone and differ in the alkyl substitution position. BTCN-O and BTCN-M demonstrate similar optical absorption spectra in solution and molecular energy levels in a solid film. BTCN-O forms an ordered lamellar packed structure with compact π-π stacking, while BTCN-M only demonstrates a weak π-π stacking effect in solid film. We also investigated their photovoltaic properties by blending each with a polymer donor, PBDB-T, and a fullerene acceptor, PC71BM, and found that the electron donating and accepting abilities of BTCNO and BTCN-M are exactly opposite. According to the results obtained from a variety of analytical methods, we can infer that for the planar A-D-A SMs, the steric hindrance caused by the non-conjugated alkyls in their central units plays a critical role that affects their electron donating and accepting properties. More specifically, the A-D-A molecules that have low steric hindrance in their central units, which allows ordered lamellar packing and compact π-π stacking in the solid film, can act as an electron donor in OPV device, and the molecules that have high steric hindrance for intermolecular ππ interactions in their central units tend to act as electron acceptors. Overall, this work provides a new perspective in the molecular design of organic photovoltaic materials.

The electronic properties of organic semiconductors are highly tunable with changes in their chemical structure. Modulation of the electron donating and accepting properties of organic semiconductors is of great importance in applications that convert incident light into electricity or electric signals such as organic photovoltaic cells (OPVs) or photodetectors.1-9 It is well-known that in organic semiconductors, the incident light generates electron-hole pairs called excitons, which are tightly bound by Coulomb interactions. To realize efficient exciton dissociation to generate more free charges in these devices, two organic semiconductors are often used to construct the photoactive layers: one serves as an electron donor, and the other serves as an electron acceptor. It has been well recognized that the difference between their molecular energy levels, i.e., the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), provide the driving force for charge generation.10-14 Therefore,

from the aspect of molecular design, the modulation of the HOMO and LUMO levels has been deemed the main strategy to tune the electron donating and/or accepting properties of organic semiconductors. With rapid progress in the OPV field in recent years, increasingly more organic semiconductors have been designed and successfully applied in the fabrication of highly efficient OPV devices.15-23 Small-molecule semiconductors (SMs) based on an A-D-A structure, where D is an electron-rich unit and A is an electron-deficient unit, can act as either an electron donor or acceptor in OPV devices.24-35 For example, Wei et al. synthesized an A-D-A-type small molecule BTID-2F by using thiophene-substituted benzodithiophene as the electron-rich cores, 2-(thiophen2-yl)thieno [3,2-b’]thiophene as π-bridges, and 4,7difluoro-1H-indene-1,3(2H)-dione as the electrondeficient end-capping groups; the BTID-2F worked

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perfectly as electron donors through blending with PC71BM in OPV devices.25 Conversely, Zhan et al. reported a series of fused-ring electron acceptors (FREAs) based on fused-ring electron donor units, such as indacenodithiophene (IDT) and indacenodithieno[3,2-b]thiophene (IT), flanked with two compact strong electron acceptor units such as 2-(3-oxo-2,3-dihydro-1H-inden-1ylidene)malononitrile (DCI), and these molecules proved to be excellent electron acceptors when blended with polymer donors.36,37 To date, high power conversion efficiencies (PCEs) have been achieved by utilizing SMs as electron donors or acceptors in these devices, which indicates that the study of this class of molecules is of great importance to the OPV field.38-42 However, from the molecular design point of view, it still remains unclear which part of the chemical structure in these A-D-A molecules determines their electron donor or acceptor properties. Scheme 1. Molecular structures of DRCN7T and ITIC.

By studying the chemical structures of the A-D-A donors and acceptors (Scheme 1), we can find some hints for answering the above questions. For example, DRCN7T and ITIC,33,40 a donor and an acceptor that yielded very high device PCEs, both molecules contain planar conjugated backbones, while the geometries of their nonconjugated alkyl chains are distinctly different. For DRCN7T, the non-conjugated alkyls are linked with the conjugated backbone through CH2 groups. As the alkyl groups are identical in length and have low steric hindrance for intermolecular π-π stacking, the DRCN7T molecules can form highly ordered lamellar packing and strong intermolecular π-π interactions.33 However, for ITIC, the non-conjugated alkyl groups are linked to the 4and 9-position of the IT units, which inevitably results in high steric hindrance effect for intermolecular π-π stacking. It has been observed that ITIC has high steric hindrance for intermolecular π-π stacking and threedimensional molecular packing is formed in the ITIC film through local intermolecular π-π stacking between the end groups.43 From these results, we can infer that the electron donating and accepting properties of the A-D-A molecules may be determined by the intermolecular packing mode. If this assumption is correct, an A-D-A donor should be changed to an A-D-A acceptor by tuning

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the steric hindrance of the non-conjugated alkyl groups in their core units. In this contribution, we designed two A-D-A SMs named BTCN-O and BTCN-M for photovoltaic applications in which the side groups present a different steric hindrance for intermolecular packing and are attached to an identical central unit. As BTCN-O and BTCN-M have the same conjugated backbone, they have very similar optical absorption spectra and molecular energy levels. The grazing-incidence wide angle X-ray scattering (GIWAXS) results demonstrate that the two molecules have very different packing properties in the aggregation state. Each molecule was blended with an electron acceptor, PC71BM, and a well-known polymeric donor, PBDBT,44 to construct the photoactive layers in OPV devices. The photovoltaic results clearly show that BTCN-O acts as a typical electron donor material, whereas BTCN-M acts as an electron acceptor in photovoltaic devices, suggesting that the aggregation structure determined by the steric hindrance of the alkyl groups significantly affects the electron donating and accepting properties of the AD-A molecules, which suggests a new strategy for the molecular design of OPV materials. Here, the two newly designed A-D-A SMs have identical conjugated backbones, as shown in Figure 1a, in which 4,8-bis-thiophenesubstituted benzo[1,2-b:4,5-b’]dithiophene (BDT-T) is used as the central electron-rich units and DCI is linked with BDT-T through a thiophene and used as the electron-deficient end groups. To ensure the solubility of the materials in organic solvents, alkyl groups are introduced into the BDT-T central units and the thiophene bridges. The only difference between the two molecules is that for BTCN-O, two octyls are attached to the 4- and 5-position of the thiophene units in BDT-T, while the octyl groups are introduced onto the 3- and 5-position in BTCN-M. This subtle difference should have a slight influence on the molecular energy levels of the two molecules but will cause a significant change in the twisting energy barriers of the thiophene in BDT-T.

RESULTS AND DISCUSSION Quantum chemistry calculation. First of all, the molecular geometries were optimized by density functional theory (DFT) using the ωB97XD functional and 6-31G** basis set. Based on the optimized geometries, the frontier molecular energy levels were then obtained at the same level of theory as the geometry optimizations while the range separation parameter (ω) of the functional was optimally tuned by a gap fitting procedure.45-47 The gap fitting of the ω values was plotted in Figure S1 (Supporting Information). The conjugated backbone of BTCN has four isomers (Figure S2) due to the rotation around the double and/or single bonds that link the end groups and bridge thiophenes. Here, the isomer with the lowest total energy was used for calculations. As demonstrated in Figure 1b, the non-substituted BTCN backbone has a planar geometry, and the LUMO and HOMO levels are -2.22 and -6.90 eV, respectively. Owing to the electron donating effect of the alkyl groups, both the HOMO and LUMO


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Figure 1. (a) Chemical structures of the BTCN, BTCN-O, and BTCN-M. (b) DFT- Chemical strucoptimized molecular geometries of simplified structures, and the HOMO and LUMO levels obtained with further tuned range-separation parameter of the functional. levels are elevated for BTCN-O and BTCN-M. In addition, although these molecules have similar optimal geometries of the conjugated backbones, the dihedral angle between the BDT and the side thiophenes in BTCN-O is smaller than that in BTCN-M, i.e., 59o versus 70o. Since there is no electronic distribution on the side thiophenes, the LUMO levels of these two molecules are very similar to each other. In contrast, the HOMOs can be delocalized over the thiophenes, thus the smaller twist in BDT-T brings about a slightly higher HOMO level for BTCN-O with respect to BTCN-M, implying that the former may have a slightly smaller band gap compared to the latter. Furthermore, potential energy surface scan was performed by DFT at the ωB97XD/6-31G** level to compare the twisting energy barriers of the BDT-T units in BTCN-O and BTCN-M. As shown in Figure 2, the twisting barrier in BTCN-O is 35 kJ/mol, while that in BTCN-M reaches 110 kJ/mol. Moreover, when a dihedral angle of 0o or 180o is adopted, as shown in the insert in Figure 2, the BDT unit in BTCN-O maintains a planar geometry structure, while the planar structure of the BDT-T unit in BTCN-M is destroyed. Figure S3 shows the geometric structures of BDT-T-M scanned at different dihedral angles between thiophene and BDT units. This result implies that the thiophene unit of BDT-T in BTCN-O can be twisted much more freely than those in BTCN-M. In addition, the twisting energy barrier in BDT-T-M is so high that two conformational Scheme 2. Synthetic routes of BTCN-O and BTCN-M.

isomers can be observed in the synthesis of BTCN-M. From our calculated results, we can infer that the alkyl substituents of the central units of these two SMs cause significant differences in the steric hindrance for intermolecular interactions. Hence, the intermolecular packing structures for BTCN-O and BTCN-M in the aggregation state should be very different.

Figure 2. Potential energy surface scan of the BDT-T units in BTCN-O and BTCN-M calculated at the DFTωB97XD/6-31G** level. Insert: the geometries of the BDTT units in BTCN-O and BTCN-M with a dihedral angle of 180o.



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Reagents and conditions: (i) n-BuLi, THF, -78°C, 1h, then 1-Bromooctane, 60°C, overnight; (ii) CHCl3, NBS, R.T., overnight; (iii) LDA, THF, -78°C, 1 h, then R.T., 5h, H2O; (iv) n-octylMgBr, Ni(dppp)Cl2, THF, 0°C, 0.5 h, then 80°C, overnight; (v) LDA, THF, -78°C, 1 h, DMF, overnight; (vi) n-BuLi, THF, 0°C, 1 h, then BDT, 50°C, 2 h, then SnCl2•2H2O, 2 h; (vii) n-BuLi, THF, -78°C, 1 h, then (CH3)3SnCl, R.T., 2 h; (viii) Pd(PPh3)4, toluene, 110°C, 24 h; (ix) Piperidine,CHCl3, 60°C, 16 h. Synthesis of BTCN-O and BTCN-M. The synthetic routes of two molecules are demonstrated in Scheme 2. The alkyl groups were introduced onto different positions of the thiophene to obtain 2,3-dioctylthiophene (1) and 2,4dioctylthiophene (4). Then, the two dioctylthiophenes were introduced in the 4- and 8position by a well-known method to synthesize compounds 6 and 9.48 It is worth noting that for compound 9, two conformational isomers can be observed by thin-layer chromatography, but the proton chemical shifts of these two isomers are identical in 1H nuclear magnetic resonance (NMR) measurements. The two isomers don’t have implications on our research so that we used them without separation. The DCI units were introduced by a Knoevenagel reaction according to a reported method.49 The detailed synthetic methods of BTCN-O and BTCN-M are provided as Supporting Information (SI). The thermal stability of the two molecules was evaluated by thermogravimetric analysis (TGA), as shown in Figure S4. The thermal decomposition temperatures (Td), defined at 5 % weight loss, all exceed330 °C, indicating that the two SMs display good thermal stability. UV-Vis absorption spectra and electrochemical properties of BTCN-O and BTCN-M. The normalized

UV-Vis absorption spectra of BTCN-O and BTCN-M in solution and thin films are shown in Figure 3a. The corresponding parameters are summarized in Table 1. In chloroform solution, the main absorption peaks of BTCNM and BTCN-O are very similar, with slightly red-shifted from 622 nm to 626 nm, implying that the electronic structures of these two molecules have no big difference without considering the intermolecular interaction. In thin films, the BTCN-O displays substantially red-shifted absorption peak by 90 nm, while the main absorption peak of BTCN-M is red-shifted by 40 nm, suggesting the different packing motifs of BTCN-O and BTCN-M in solid states. The absorption onsets of BTCN-O and BTCN-M locate at 812 and 763 nm, respectively, corresponding to the optical band gaps of 1.53 and 1.63 eV. Cyclic voltammetry (CV) was adopted to measure the pdoping and n-doping behavior of the two materials, from which the ionization potential (IP) and electron affinity (EA) can be derived.50 The measurement was conducted on electrochemical workstation using glassy carbon discs as the working electrode, platinum wire as the counter electrode, and Ag/AgCl electrode as the reference electrode with a scanning rate of 20 mV/s in a 0.1 M tetrabutylammonium hexafluorophosphate


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Figure 3. (a) Normalized UV-Vis absorption spectra of BTCN-O and BTCN-M in a chloroform solution and as thin films. (b) Cyclic voltammetry curves of the BTCN-O and BTCN-M films on a glassy carbon electrode in a 0.1 M Bu4NPF6 acetonitrile solution at a scan rate of 20 mV/s, calibrated by an Fc/Fc+ redox couple. Table 1. Optical parameters, IP, EA and charge mobilities of BTCN-O and BTCN-M. opt

Molecule

Sol. λmax (nm)

Film λmax (nm)

ε film 4 -1 (×10 cm )

Film λedge (nm)

Eg (eV)

IP (eV)

EA (eV)

BTCN-O

626

716

5.12

812

1.53

-5.59

-3.95

BTCN-M

622

662

6.40

763

1.63

-5.69

-3.95

a

μh -1 -1 (cm V s ) -4 2.24×10 -4 ± 0.52×10 -5 4.92×10 -5 ± 0.42×10 2

a

μe -1 -1 (cm V s ) -7 8.13×10 -7 ± 0.31×10 -5 2.91×10 -5 ± 0.38×10 2

a

The charge mobility was measured via the space-charge-limited-current (SCLC) method and the average values were obtained over 5 devices.

(Bu4NPF6) solution. Here, the redox couple of ferrocene (Fc/Fc+) was used to calibrate the CV plots. As shown in Figure 3b, BTCN-O and BTCN-M show almost the same n-doping onset values at approximately -0.85 V, which corresponds to a EA of -3.95 eV; the p-doping onset of BTCN-O is 0.79 V, which corresponds to a IP of -5.59 eV, 0.10 eV higher than that of BTCN-M. GIWAXS Measurement. GIWAXS was used to investigate the crystallinity structures which is crucial for device performance.51 Here, two solid films of the two molecules were prepared by spin-coating their solutions in chloroform (10 mg/ml). Figure 4a and 4b show the 2DGIWAXS patterns of the BTCN-O and BTCN-M films. The BTCN-O film presents quite a complicated pattern, which implies that highly ordered structures are formed. Based on the 2D-patterns, it can be seen that BTCN-O has clear (100), (200) and (300) reflections in the out-of-plane (OOP) direction and a narrow strong (010) reflection pattern in the in-plane (IP) direction, implying that a highly ordered structure with an edge-on dominant molecular orientation is formed. However, for BTCN-M, the (100) and (010) reflections appear in the IP and OOP directions, respectively, and the reflection patterns are quite broad and weak, suggesting that BTCN-M has low crystallinity and tends to form a face-on molecular orientation. The OOP and IP GIWAXS profiles of the two films are shown in Figure 4c to give more detailed information of the crystallinity structures. The (100) lamellar packing peaks of BTCN-O are shown at q = 0.28 Å-1, corresponding to a d-spacing of 22.4 Å, while the (100) peak of BTCN-M moves slightly to the right with q = 0.29 Å-1 (21.6 Å). The coherence length (CL) of BTCN-O and BTCN-M lamellar packing in the IP direction are

calculated (via the Scherrer equation52) to be 23.8 nm and 5.8 nm, respectively. In the OOP direction, the CL of the (010) peak of BTCN-O is 26.6 nm, while the (100) peak of BTCN-M is too weak to perform the Gauss function fitting. The pronounced (010) peak of BTCN-O is observed in both the IP and OOP directions with q = 1.77 Å-1, corresponding to a d-spacing of 3.55 Å. The π-π stacking CLs of BTCN-O in the IP and OOP directions are calculated to be 8.3 and 6.6 nm, respectively. For BTCNM, the (010) peak is only observed in the OOP direction at q = 1.74 Å-1 (~3.61 Å). The π-π stacking CL of BTCN-M is calculated to be 2.1 nm. All of these results confirm that the molecular packing of BTCN-O is much stronger than that of BTCN-M.

Figure 4. Two-dimensional (2D)-GIWAXS patterns of neat (a) BTCN-O and (b) BTCN-M films. (c) The corresponding intensity profiles in the out-of-plane (OOP) and in-plane (IP) directions.



Then, it is now possible to surmise why the two molecules can form the different aggregation structures. For BTCN-O, compact and strong intermolecular π-π stacking and highly ordered lamellar packing can be formed in the central core units because the alkyls in this molecule have low steric hindrance for intermolecular π-π interaction of the entire conjugated chains. For BTCN-M, as the dioctylthiophene units in BDT are highly twisted, the intermolecular π-π stacking can only occur between the end groups in two adjacent molecules; under such a situation, the intermolecular π-π interaction is not strong and thus gives a larger π-π distance, and ordered lamellar packing cannot be formed. Photovoltaic behaviors of BTCN-O and BTCN-M in OPV devices. To investigate the photovoltaic behaviors of the two molecules as the electron donor and acceptor in OPV devices, we selected two materials, PC71BM as an acceptor53,54 and PBDB-T as a donor polymer,41,44 to blend with BTCN-O and BTCN-M, respectively. The molecular energy level digram of the four materials is demonstrated in Figure S5. As is shown, the HOMO levels and the LUMO levels of both BTCN-O and BTCN-M are more than 0.3 eV lower compared to those of PBDB-T, meaning that both of them could work as electron acceptors in the combinations with PBDB-T as the electron donor from the aspect of molecular energy levels.55 Similarly, they can also be used as electron donors when blended with PC71BM. The devices were fabricated with a reverse structure, i.e., ITO/ZnO/Active layer/MoO3/Al,

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and the detailed fabrication procedures are provided in the experimental section. Based on these conditions, we can obtain a series of OPV devices with four different photoactive layers. The photovoltaic properties of the devices were characterized under illumination of simulated solar light, AM1.5G, 100 mW/cm2. The representative current density-voltage (J-V) curves are displayed in Figure 5a, and the corresponding key photovoltaic parameters are summarized in Table 2. In comparison with the BTCN-O:PBDB-T device, the BTCNM:PBDB-T device provides a similar open circuit voltage (Voc) but significantly higher short circuit current (Jsc) and fill factor (FF), leading to a much higher PCE of 5.89% versus 1.62%. When blended with PC71BM, BTCN-O and BTCN-M also demonstrated very different photovoltaic behaviors. The BTCN-O:PC71BM device showed a high Voc of 0.97 V, a Jsc of 11.68 mA/cm2 and an FF of 0.59, resulting in a PCE of 6.68%, which is quite a good result for OPV devices based on a small-molecule active layer materials. However, for the BTCN-M:PC71BM device, the Jsc and FF are very low, resulting in an overall PCE of only 0.29%. Considering the photovoltaic results of the four devices, we can conclude that as an electron acceptor after blending with PBDB-T, BTCN-M works better than BTCN-O, while as an electron donor after blending with PC71BM, BTCN-O displays a superior photovoltaic performance than BTCN-M. That is, the electron donating and accepting abilities of BTCN-O and BTCN-M are opposite; i.e., BTCN-O is a good donor, and BTCN-M is a good acceptor.

Figure 5. (a) Current density-voltage (J-V) curves of BTCN-O:PC71BM, BTCN-O:PBDB-T, BTCN-M:PC71BM and BTCNM:PBDB-T based devices under 1 sun illumination. (b) EQE spectra of the corresponding photovoltaic devices. (c) PL spectra of PC71BM, BTCN-O, BTCN-M, BTCN-O:PC71BM and BTCN-M:PC71BM films; the samples were excited at 650 nm. (d) PL spectra of PBDB-T, BTCN-O, BTCN-M, BTCN-O:PBDB-T and BTCN-M:PBDB-T films; the samples were excited at 740 nm.


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Table 2. Photovoltaic parameters of the BTCN-O:PBDB-T, BTCN-M:PBDB-T, BTCN-O:PC71BM and BTCNM:PC71BM devices with a reverse structure of ITO/ZnO/Active layer/MoO3/Al. Active layer BTCN-O:PBDB-T BTCN-M:PBDB-T BTCN-O:PC71BM BTCN-M:PC71BM a

Voc (V) 0.95 0.95 ± 0.01 0.98 0.98 ± 0.00 0.97 0.97 ± 0.01 1.10 1.08 ± 0.01

2

Jsc (mA/cm ) 5.03 5.04 ± 0.04 12.03 11.91 ± 0.09 11.68 11.76 ± 0.03 0.62 0.61 ± 0.02

FF 0.34 0.34 ± 0.00 0.50 0.49 ± 0.01 0.59 0.58 ± 0.01 0.43 0.40 ± 0.03

a

PCE (%) 1.62 1.62 ± 0.01 5.89 5.72 ± 0.11 6.68 6.56 ± 0.07 0.29 0.26 ± 0.03

The average values were obtained over 10 devices.

Figure 5b compares the external quantum efficiency spectra (EQE) of the four devices. The BTCN-O-based device demonstrated a broad EQE response from 300 to 800 nm. Notably, the BTCN-O:PC71BM-based device showed a greater response at longer wavelengths in the range of 700-800 nm than the device based on BTCNM:PBDB-T. This behavior can be attributed to the contribution of BTCN-O. The integrated EQEcurves give calculated Jsc values of 4.78, 11.41, 11.05, and 0.45 mA/cm2 for BTCN-O:PBDB-T, BTCN-M:PBDB-T, BTCN-O:PC71BM and BTCN-M:PC71BM, respectively. These results are in agreement with the photovoltaic measurements. The photocurrent density (Jph) (Jph = JL-JD, where JL and JD are the current density under illumination and in the dark) as a function of the effective voltage (Veff) (Veff = V0-Va, where V0 is the voltage at Jph = 0, and Va is the measured voltage under a different current density) of the optimal devices was characterized to estimate the overall efficiency of exciton dissociation (Pdiss) (Figure S7). The Pdiss was calculated using the formula Pdiss = Jph/Jph,sat. Under the short-circuit condition, the BTCN-O:PC71BM device showed a Pdiss of 78.6 %, and the BTCN-M:PBDB-T exhibited an even higher Pdiss of 92.1 %. However, the Pdiss values in the BTCN-M:PC71BM and BTCN-O:PBDB-T devices were 35.2 and 43.1 %, respectively. These results imply that the exciton dissociation efficiency in these devices is much lower than that in the BTCN-M:PBDB-T and BTCN-O:PC71BM devices. Photoluminescence quenching effects of the blends. Photoluminescence (PL) was performed to investigate exciton dissociation in the of blends of BTCN-O:PC71BM, BTCN-M:PC71BM, BTCN-O:PBDB-T, and BTCN-M:PBDBT. The films are prepared by the same method as used for device fabrication. The UV-vis absorbance of neat and blend films are shown in Figure S6. To investigate the donor character of BTCN-O and BTCN-M, the fluorescence spectra were measured in the range of 660-900 nm with exciting wavelength at 650 nm. As shown in Figure 5c, the PL of BTCN-O almost fully quenched by PC71BM, whereas the PL of BTCN-M can only be partially quenched, indicating that the exciton dissociation in the blend of BTCNO:PC71BM should be more efficient than that in the blend of BTCN-M:PC71BM. For studying acceptor character of these two SMs, the exciting wavelength at 740 nm was chosen (Figure 5d). In the BTCN-M:PBDB-T film, the PL quenching effect is much stronger than that in the BTCN-

O:PBDB-T film, supporting that the exciton dissociation in the latter is less efficient. These results are in good consistent with device performance. Hole and electron mobilities of the neat and the blend films. The space-charge-limited-current (SCLC) method was employed to measure the hole and electron mobilities (μh and μe) of the neat and blend films (Figure S8), and the electron and hole only devices were fabricated and characterized according to reported methods.56 The neat BTCN-O film has a μh of 2.24×10-4 cm2 V-1 s-1 and a μe of 8.13×10-7 cm2 V-1 s-1. In comparison with BTCN-O, the μh of BTCN-M (4.92×10-5 cm2 V-1 s-1) is one order of magnitude lower, but its μe (2.91×10-5 cm2 V-1 s-1) is significantly higher by up to two orders of magnitude. The results indicate that BTCN-O behaves more similarly to a p-type organic semiconductor, while BTCN-M is ambipolar. The BTCN-O:PC71BM film has comparatively high and symmetric μh (8.0×10-5 cm2 V-1 s-1) and μe (2.6×10-5 cm2 V-1 s-1), which may be the reason for the good FF of the OPV device. In the blend of BTCNM:PC71BM, μh (1.3×10-6 cm2 V-1 s-1) and μe (1.9×10-6 cm2 V-1 s-1) are much lower than those in the neat films of BTCN-M and PC71BM; however, in the BTCN-M:PC71BM-based OPV device, the inferior PCE not only is due to the low mobilities but also can be attributed to the problem in exciton dissociation, as discussed in the PL measurements. The two blends of BTCN-O:PBDB-T and BTCN-M:PBDBT demonstrated similar carrier transport properties, i.e., their μh and μe are approximately 1.0×10-5 and 2.0×10-7 cm2 V-1 s-1, respectively. Therefore, the higher PCE in the BTCN-M:PBDB-T device compared to the BTCN-O:PBDBT device is predominantly due to the more efficient exciton dissociation. Overall, it must be recognized that the correlation between carrier mobility and photovoltaic performance is quite complicated, with the results shown here only demonstrating that these two new small molecules display very different charge transport characters, i.e., BTCN-O behaves more like a p-type material, while BTCN-M is ambipolar, and as these two molecules share an identical conjugated backbone, ascribed to the different structures formed in the aggregation state. Phase separation morphologies of the blend films. The phase separation morphologies of the four blend films were investigated by atomic force microscopy (AFM)



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Figure 6. Tapping-mode AFM (a, d, g and j) topography and (b, e, h and k) phase images and (c, f, i and l) TEM images of (a-c) BTCN-M:PBDB-T, (d-f) BTCN-O:PBDB-T, (g-i) BTCN-M:PC71BM, and (j-l) BTCN-O:PC71BM blend films. and transmission electronic microscopy (TEM). As shown in Figure 6a and Figure 6b, the BTCN-M:PBDB-T film exhibited a uniform and smooth surface with a Rq of 1.31 nm.The TEM image shown in Figure 6c reveals that this film is also quite uniform in the bulk, which means that the bicontinuous charge transport channels are not fully evolved in this blend, which may cause low carrier transport ability for holes and/or electrons. For the BTCN-O:PBDB-T film, the phase separation effect is much stronger than that observed in the BTCN-M:PBDBT film, i.e., the surface roughness is 4.10 nm (Figure 6d) and large aggregates are observed in the bulk material (Figure 6f). For the two films containing PC71BM, the phase separated morphologies differ significantly. For the BTCN-M:PC71BM film, granular aggregations with a size of tens of nanometers can be observed on the top surface (Figure 6g and 6h), and these aggregates can also be distinguished in the bulk and further form over 100 nm domains (Figure 6i); for the film of BTCN-O:PC71BM, much smaller granular aggregates are observed on the top surface (Figure 6j and 6k), as is a network-like nanoscale phase separated morphology in the bulk material (Figure 6l). Furthermore, on the basis of the above observations, we were inspired to make a device based on BTCN-O:BTCNM. To make comparisons, devices based on the neat BTCN-O and BTCN-M were also fabricated and characterized. As shown in Figure 7 and Table 3, the two devices based on the neat small molecules exhibit very low PCE, i.e., 0.05% for the BTCN-O-based device, and

0.01% for the BTCN-M-based device, which are at the same level for the OPV devices based on one organic semiconductor. However, by blending the two materials together, a Jsc of 0.39 mA/cm2 and an FF of 0.38 were obtained, resulting in a PCE of 0.17%. Although the photovoltaic performance of the BTCN-O:BTCN-M device is very low, it can still be viewed as a successful device. Moreover, as shown in Figure S9, the PL of the BTCN-O and BTCN-M is weakly quenched by each other, indicating that weak exciton dissociation still occurs at the interface between BTCN-O and BTCN-M.

Figure 7. J-V curves of the BTCN-O, BTCN-M, BTCNO:BTCN-M and BTCN-O:BTCN-M-4F devices under 1 sun illumination.


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Table 3. Photovoltaic parameters of the BTCN-O, BTCN-M, BTCN-O:BTCN-M, BTCN-O:BTCN-M-4F devices with a reverse structure of ITO/ZnO/Active layer/MoO3/Al. Active layer BTCN-O BTCN-M BTCN-O:BTCN-M BTCN-O:BTCN-M-4F a

2

a

Voc (V)

Jsc (mA/cm )

FF

PCE (%)

1.07 1.04 ± 0.02 1.09 1.04 ± 0.03 1.12 1.11 ± 0.01 1.03 1.02 ± 0.01

0.15 0.15 ± 0.02 0.03 0.03 ± 0.01 0.39 0.35 ± 0.05 2.81 2.72 ± 0.09

0.32 0.32 ± 0.01 0.30 0.30 ± 0.01 0.38 0.36 ± 0.03 0.35 0.34 ± 0.01

0.05 0.05 ± 0.00 0.01 0.01 ± 0.00 0.17 0.14 ± 0.02 1.01 0.94 ± 0.05

The average values were obtained over 5 devices.

As shown in Figure3b, both the IP and EA of BTCN-O are approximately aligned with those of BTCN-M. Hence, the driving force for charge separation is negligible, and under such circumstances, it is difficult to determine which material is working as a donor and which is working as an acceptor. Here, we introduced two fluorine atoms into the end groups of BTCN-M and synthesized another molecule named BTCN-M-4F (Scheme S1); its IP and EA are downshifted by approximately 0.10 eV compared to BTCN-M (Figure S10). By using a blend of BTCN-O:BTCN-M-4F as the active layer, the PL quenching effect in the blend was enhanced (Figure S9), and the Jsc of the device significantly improved to 2.81 mA/cm2, resulting in a PCE of 1.01%. Therefore, we can conclude that in the blend of BTCN-O:BTCN-M, BTCN-O and BTCN-M act as the electron donor and the electron acceptor, respectively.

CONCLUSION In summary, we designed and synthesized two novel small-molecule photovoltaic materials, BTCN-O and BTCN-M, and investigated their photovoltaic properties by blending each of them with a polymer donor, PBDB-T, and a fullerene acceptor, PC71BM. BTCN-O and BTCN-M are very similar in chemical structure, only differing in the substitution position of the alkyl groups attached to their conjugated cores. As a result, properties that are determined by their conjugated backbones, including electronic absorption and molecular energy levels, were similar for BTCN-O and BTCN-M. However, properties that are affected by the aggregation structures formed by the two molecules are very distinct, i.e., the redshift of their absorption spectra from solution to solid film, the mobilities and the electron donating and accepting properties displayed in OPV devices. According to the results shown in this work, we can clearly see that for molecules with a planar conjugated backbone with an AD-A structure, the steric hindrance for intermolecular π-π interaction caused by the non-conjugated alkyls in the central units plays a critical role in the electron donating and accepting properties. We can further infer that A-D-A molecules that have low steric hindrance for intermolecular π-π interactions between their central units form ordered lamellar structures, and the compact π-π stacking in the solid film can act as an electron donor in a bulk heterojunction OPV device, while the A-D-A molecules that have high steric hindrance, which disrupts

the intermolecular π-π interactions between their central units, tend to act as electron acceptors. Overall, this work correlates the chemical structure of the A-D-A semiconductors with their aggregation structure in the solid state and their electron donating and accepting properties in OPV devices and thus provides a new perspective for the molecular design of organic photovoltaic materials.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. 1 Synthetic routes of BTCN-M-4F, experimental methods, H 13 and C NMR spectra, theoretical calculations of the possible regioisomer molecular models, thermogravimetric analysis (TGA) plots of BTCN-O, BTCN-M and BTCN-M-4F, photocurrent analysis of the corresponding devices, hole and electron mobility curves, fluorescence (PL) spectra, normalized absorption spectra and energy levels of BTCN-M-4F. (PDF)

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] *E-mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors acknowledge the financial support of NSFC (9163301, 21734008, 21325419, 91333204and 51373181), and the Chinese Academy of Sciences (XDB12030200, KJZD-EW-J01).

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